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In the UK, 12,000 patients die from metastatic breast cancer annually. There is therefore an urgent need to identify the molecules that cause metastasis. Recent work has revealed a role for the RUNX family of transcription factors in the development of metastatic breast cancer. The RUNX proteins form active transcription factor complexes when bound by the heterodimeric partner CBFβ to regulate the expression of metastatic genes. Previous work from our laboratory has demonstrated that knockdown of CBFβ resulted in a decreased invasion capacity of the metastatic breast cancer cell line MDA-MB-231. Three-dimensional culture of MDA-MB-231 cells revealed that loss of CBFβ induces a mesenchymal to epithelial transition (MET). The aim of this project was to determine the role of the RUNX/CBFβ complex in maintaining the mesenchymal phenotype of metastatic breast cancer cells. The data presented show that the phenotype changes were accompanied by changes in EMT marker-gene expression, including Snai2, MMP9, and MMP13. Induction of CBFβ in the CBFβ-knockdown cells remarkably restored both the invasive capacity and the mesenchymal phenotype. Further analysis revealed that maintenance of the mesenchymal phenotype was dependent upon both CBFβ-partner proteins, RUNX1 and RUNX2. Taken together the data presented in this thesis demonstrate that RUNX/CBFβ complexes drive the epithelial to mesenchymal transition (EMT) in breast cancer cells. These findings are likely to be important in the development of potential therapies to inhibit the metastatic spread of breast cancer.